Recently, with respect to steel sheets for automobiles, the strength of steels used therefor has been gradually increased as economy in automotive fuel consumption as well as passenger safety during automobile collisions are increasingly required, and simultaneously, steel sheets having a higher level of formability are required due to trends for complexity and integration in formed automotive parts. In addition, since the weight of automotive fuel systems, including batteries, is expected to greatly increase in comparison to current internal combustion engine fuel systems, according to the emergence of automobiles using new fuels replacing petroleum, there is a need to develop a lightweight material able to significantly reduce the weight of an automotive body.
Since steel has significantly better strength and ductility than of aluminum and magnesium, and production costs thereof are also relatively low, weight reduction in an automotive body is currently achieved by decreasing a thickness of a typical high-strength and high-ductility steel sheet. However, in the case that an amount of weight reduction required for future automobiles employing replacement fuels is not satisfied, the use of a non-ferrous lightweight metal, such as aluminum or magnesium, is inevitable.
Accordingly, the development of steels having a decreased specific gravity in comparison to typical steels through adding aluminum (Al), a lightweight element, to steel, has been undertaken. Such lightweight steels are typically categorized as multiphase, austenitic, and ferritic steels, and a typical technique for ferritic lightweight steel is disclosed in Japanese Patent Laid-Open Publication No. 2005-273004. The technique is for manufacturing ferritic lightweight steels by adding 2.0% to 10.0% of Al to ultra-low carbon steel, but an effect of reducing specific gravity may be low and tensile strength×elongation (TS×El) may be significantly low. Another technique is disclosed in Japanese Patent Application Laid-Open Publication No. 2006-176843. The technique is for manufacturing lightweight steels by including 0.8% to 1.2% of carbon, adding 10% to 30% of manganese (Mn) and 8% to 12% of Al, and including as low an amount of (Fe,Mn)3AlC as possible, but drawability may be low. Another technique is disclosed in Japanese Patent Application Laid-Open Publication No. 2006-149204. The technique secures stiffness and ductility through controlling texture, but tensile strength is only on the level of 400 MPa. Some steels having a tensile strength of 620 MPa are disclosed, but elongation thereof is only 25%, and 3% or more of the height of a drawn cup may be cut because of high earing in the form of mountain in the drawn cup during processing, such as drawing. Another technique is disclosed in Japanese Patent Application Laid-Open Publication No. 2003-355229. The technique is for manufacturing steels including 0.01% to 5% of carbon (C), 0.01% to 5.0% of Mn, and 3% to 10% of Al, and reduces specific gravity by the addition of a large amount of Al. However, heat treatment (including cooling) temperature and cold reduction rates may be limited after hot rolling due to the limitations of hot rolling and cold rolling cracks, and although a value of TS×El is 10,000 MPa % or more, there may be a limitations in the processing of automotive parts, because the value of TS×El may not reach a level of advanced commercial high-strength steel at 16,000 MPa %.